1. Field of the Invention
This invention relates to a plastic material molding apparatus and more particularly to a plastic material molding apparatus which has a material holding mechanism and a material metering and delivering mechanism.
2. Description of the Prior Art
FIG. 1 shows a conventional plastic material molding apparatus with a plastic material holding mechanism and a metering and delivering mechanism. In the figure, reference numeral 1 represents a hopper for supplying plastic material; 2 a plastic material fluidizing mechanism; 3 a drive motor; 4 a metering and delivering mechanism; 5 a material holding mechanism installed, through valves 6, 7, between the fluidizing mechanism 2 and the metering and delivering mechanism 4; 8 a cylinder for the material holding mechanism 5; 9 and 10 cylinder pistons on the material side and on the pressure chamber side respectively; 11 a piston rod connecting these pistons; 12 a pressure fluid supply port in the cylinder 8 that opens to the pressure chamber of the cylinder; 13 a pressure detector for detecting the pressure at the material inlet of the cylinder 8; 14 molds for successively accepting the kneaded material intermittently discharged out of the metering and delivering mechanism 4; 15 a conveyor; and 16 a motor for the delivering mechanism 4.
In the above plastic material molding apparatus, the plastic material is heated and fluidized by the fluidizing mechanism 2 and introduced through the valve 6 into the material holding mechanism 5. The valve 7 is opened as needed and pressurized air is supplied into the pressure chamber in the cylinder 8 to push the material in the cylinder 8 into the metering and delivering mechanism 4. The motor 16 is then driven to discharge the metered material intermittently onto the molds 14 on the conveyor 15.
In such plastic material molding apparatuses, the material is introduced into the material holding mechanism 5. When the cylinder 8 of the material holding mechanism 5 is filled with the material and the pressure at the material inlet of the cylinder 8 becomes higher than the pressure in the pressure chamber of the cylinder 8, the increased pressure is detected by the pressure detector 13 to stop the motor 3 for the fluidizing mechanism 2.
However, when the motor 3 is stopped, the fluidizing mechanism 2 does not immediately stop due to the inertia of the motor 3 and the fluidizing mechanism 2, so that the material continues to be forced into the cylinder 8 even after the piston 10 on the pressure chamber side of the material holding mechanism 5 has contacted the rear wall of the cylinder 8. Thus, as shown in FIG. 2, the pressure at the material inlet of the cylinder 8 suddenly increases, leading to a possible failure of the cylinder and associated components.
The amount of plastic material intermittently discharged from the metering and delivering mechanism 4 is determined by the product of the revolution speed of the motor 16 and the duration of motor rotation. The time during which the motor is operated is generally set by a timer.
The timer, however, has variations in its operation time and also is affected by ambient temperatures. In addition, the motor speed greatly varies depending on the magnitude of load and supply voltage variations.
The operation time variations of the timer generally fall in the range of .+-.0.3 to .+-.2% of the maximum setting time. In practice, the timer is often used at about one third of the maximum setting time. In that case the operation time variation is three times as large as the above variation range, i.e., .+-.0.9 to .+-.6%.
The effect of the ambient temperature on the timer operation time is about .+-.2% of the maximum setting time.
Therefore, when for example a 120-second timer is set at 60 seconds, the operation time variation will be .+-.120.times.0.02=2.4 seconds; and the effect of temperature will be .+-.120.times.0.02=2.4 seconds.
Thus the total variation will be as large as .+-.4.8/60=.+-.8%
In the metering and delivering mechanism that intermittently discharges such plastic materials as hot melted polymer, variations in the weight of each shot cannot be prevented, even if the rotation of the delivery pump is controlled accurately.
The primary cause of the variations is that the water contained in the melted material is vaporized to form bubbles and thereby increases the volume of the material, so that even when the delivery pump is correctly controlled to discharge the material as accurately as possible, the material thus discharged will be lighter by as much as there are bubbles.
To solve this problem, it is most effective to suppress the formation of water vapor and this can be achieved by keeping the melted polymer pressure higher than the vapor pressure.
FIG. 3 shows the operation sequence of the material holding mechanism in a conventional plastic material molding apparatus. When the pressure detector 13 detects a pressure higher than that in the pressure chamber of the cylinder 8, the fluidizing mechanism 2 is stopped, say, 60 seconds after starting, at which time 2 liters of material for example is contained in the cylinder 8.
Then, the valve 7 is opened and a pressure fluid is supplied into the pressure chamber of the cylinder 8 to discharge about 2 liters of material contained in the cylinder 8 out into the external mechanism such as the metering and delivering mechanism 4 during the period of say 30 seconds.
Such a conventional plastic material molding apparatus has the drawback that the fluidizing mechanism 2 is operated intermittently and the operation time is as long as 90 seconds.